The recent development of electrochemical cells based on lithium containing negative electrode structures has allowed the fabrication of cells of high energy density. Cells have been obtained, which display energy densities beyond 200 Wh/l and even 250 Wh/l has been reported.
In order to reach such high energy densities, the capacity utilization of the active materials in the cell should be high. As further high rate capabilities of the electrochemical cells are sought, electrolyte phases should provide low impedance.
Traditionally, high conductivity electrolytes have been liquid electrolytes and such electrolytes are used in lithium cells of intercalation compound electrodes. The positive electrode structures are based on transition metal oxides operating at a potential close to 4V vs. Li/Li+. Negative electrode structures of carbons and graphites may be applied, which reversibly intercalate lithium at a potential close to the potential of metallic lithium. Such cells are referred to as lithium-ion cells, as the active lithium is always in its ionic form. Alternatively, negative electrode structures of alloys such as Li—Al and Li—Sn may be used. Such cells will be referred to as lithium-alloy cells. All of the above configurations provide voltages close to 4V.
As high energy density and high rate capability imply reduced safety, an important objective in the development of such electrochemical cells has been to improve the safety aspects of the use of such cells.
The main improvement in terms of safety has been the substitution of carbon or alloy structures for the pure lithium metal negative electrode structures. During operation of lithium metal based negative electrodes, dendrites form, which penetrate the cell separator and shorten the cell. Although the risk of dendrite formation cannot be completely ignored in lithium-ion or lithium-alloy cells, especially during high-rate charging, the risk is strongly reduced compared to lithium metal cells.
Another problem associated with the liquid electrolytes traditionally applied is leakage of the electrolyte. The leakage may not only lead to cell failure but penetration of the corrosive fluid may destroy the electronic device in which the cell is used.
One approach to solve these problems has been the application of solid state electrolytes and the use of such electrolytes should eliminate the risk of dendrite formation and of electrolyte leakage.
U.S. Pat. No. 5,296,318 to Bell Communication Research describes the use of a polymer electrolyte based on polyvinylidenefluoride-hexafluoropropylene copolymer, which is present in the electrolyte phase in an amount corresponding to 30-80% by weight of the electrolyte system. Electrochemical cells based on such polymer electrolytes have significantly lower conductivity than cells based on a liquid electrolyte.
U.S. Pat. No. 5,418,091 to Bell Communication Research describes a multistep, process for the application of polymer electrolytes as described in U.S. Pat. No. 5,296,318.
The above patents illustrate the problem of solid state electrolytes, and in particular of polymer electrolytes which are the best ambient temperature candidate since their conductivity is too low. Due to the low conductivity of the polymer electrolytes, the performance of the electrochemical cells in which they are applied is significantly reduced compared to liquid electrolyte cells.
U.S. Pat. No. 5,688,293 to Oliver et al describes an electrochemical cell with a gel electrolyte which comprises a solvent and a gel forming polymer. The electrolyte was applied to the electrodes and gelled by heating.
U.S. Pat. No. 5,705,084 to Joseph Kejha discloses a composite solid state or semi-solid state polymer electrolyte for batteries, capacitors and other electrochemical devices, wherein the electrolyte mixture contains polyethylene oxide, polyvinylidenefluoride/hexafluoropropylene, a salt and at least one aprotic liquid. In a preferred embodiment the electrolyte comprises polyvinylidenefluoride/hexafluoropropylene in an amount in the range 0.1-70% and polyethylene oxide in an amount in the range 0.5-70% by weight of the electrolyte system, respectively. The patent discloses the coating of electrodes with solutions of the above electrolyte, prepared at 60-90° C., and the subsequent thickening of the electrolyte upon solvent evaporation. The patent, however, does not mention anything about changes in the rheological behaviour of the electrolyte other than those brought about by evaporation of the solvent.
WO 98/28812 to Danionics discloses a lithium secondary battery comprising an immobilized electrolyte containing one or more alkali metal salts, one or more non-aqueous solvents and an immobilizing polymer, wherein the immobilizing polymer is selected from the group consisting of cellulose acetates, cellulose acetate butyrates, cellulose acetate propionates, polyvinylidene fluoride-hexafluoropropylenes and polyvinylpyrrolidone-vinyl acetates with the proviso that in the case of polyvinylidene fluoride-hexafluoropropylenes, the polymer is present in an amount of at most 12% by weight based on the weight of the salts, solvents and polymer of the electrolyte system. The specification discloses a method for the preparation of a lithium secondary battery comprising the steps of solvent mixing, salt dissolution, addition of immobilizing polymer and sandwiching of the electrode between positive and negative electrodes. However, the specification does not mention the rheological behaviour of the electrolyte.
Therefore, there is a pressing need for polymer electrolytes and process technology therefor, which will provide electrochemical cells of high performance and safety, and which can be applied by a simple, low cost process.
It is thus an object of the present invention to provide such polymer electrolytes, which display performance similar to liquid electrolytes with improved safety, and processes therefor, which are simpler than traditional processing of polymer electrolytes.